Illumination system of a microlithographic projection exposure apparatus

ABSTRACT

An illumination system of a microlithographic projection exposure apparatus includes an optical integrator that includes an array of optical raster elements. A condenser superimposes the light beams associated with the optical raster elements in a common field plane. A modulator modifies a field dependency of an angular irradiance distribution in an illuminated field. Units of the modulator are associated with one of the light beams and are arranged at a position in front of the condenser such that only the associated light beam impinges on a single modulator unit. The units are configured to variably redistribute, without blocking any light, a spatial and/or an angular irradiance distribution of the associated light beams. A control device controls the modulator units if it receives an input command that the field dependency of the angular irradiance distribution in the mask plane shall be modified.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims benefit under35 USC 120 to, U.S. application Ser. No. 13/727,903, filed Dec. 27,2012, which is a continuation of, and claims benefit under 35 USC 120to, international application PCT/EP2010/005317, filed Aug. 30, 2010.U.S. application Ser. No. 13/727,903 and international applicationPCT/EP2010/005317 are hereby incorporated by reference in theirentirety.

FIELD

The disclosure generally relates to an illumination system of amicrolithographic projection exposure apparatus and to a method ofoperating such an apparatus.

BACKGROUND

Microlithography (also referred to as photolithography or simplylithography) is a technology for the fabrication of integrated circuits,liquid crystal displays and other microstructured devices. The processof microlithography, in conjunction with the process of etching, is usedto pattern features in thin film stacks that have been formed on asubstrate, for example a silicon wafer. At each layer of thefabrication, the wafer is first coated with a photoresist which is amaterial that is sensitive to light of a certain wavelength. Next, thewafer with the photoresist on top is exposed to projection light througha mask in a projection exposure apparatus. The mask contains a circuitpattern to be imaged onto the photoresist. After exposure thephotoresist is developed to produce an image that corresponds to thecircuit pattern contained in the mask. Then an etch process transfersthe circuit pattern into the thin film stacks on the wafer. Finally, thephotoresist is removed. Repetition of this process with different masksresults in a multi-layered microstructured component.

A projection exposure apparatus typically includes an illuminationsystem that illuminates a field on the mask that may have the shape of arectangular or curved slit, for example. The apparatus further includesa mask stage for aligning the mask, a projection objective (sometimesalso referred to as ‘the lens’) that images the illuminated field on themask onto the photoresist, and a wafer alignment stage for aligning thewafer coated with the photoresist.

A desire in the development of projection exposure apparatus is to beable to lithographically define structures with smaller and smallerdimensions on the wafer. Small structures lead to a high integrationdensity, which generally has a favorable effect on the performance ofthe microstructured components produced with the aid of such apparatus.

Various approaches have been pursued in the past to achieve this aim.One approach has been to reduce the wavelength of the projection lightused to image the circuit pattern onto the photoresist. This exploitsthat fact that the minimum size of the features that can belithographically defined is approximately proportional to the wavelengthof the projection light. Therefore the manufacturers of such apparatusstrive to use projection light having shorter and shorter wavelengths.The shortest wavelengths currently used are 248 nm, 193 nm and 157 nmand thus lie in the deep (DUV) or vacuum (VUV) ultraviolet spectralrange. The next generation of commercially available apparatus will useprojection light having an even shorter wavelength of about 13.5 nmwhich is in the extreme ultraviolet (EUV) spectral range. An EUVapparatus contains mirrors instead of lenses because the latter absorbnearly all EUV light.

Another approach is to improve the illumination of the mask. Ideally,the illumination system of a projection exposure apparatus illuminateseach point of the field illuminated on the mask with projection lighthaving a well defined spatial and angular irradiance distribution. Theterm angular irradiance distribution describes how the total lightenergy of a light bundle, which converges towards a particular point onthe mask, is distributed among the various directions of the rays thatconstitute the light bundle.

The angular irradiance distribution of the projection light impinging onthe mask is usually adapted to the kind of pattern to be imaged onto thephotoresist. For example, relatively large sized features may involve adifferent angular irradiance distribution than small sized features. Themost commonly used angular irradiance distributions are referred to asconventional, annular, dipole and quadrupole illumination settings.These terms refer to the irradiance distribution in a pupil surface ofthe illumination system. With an annular illumination setting, forexample, only an annular region is illuminated in the pupil surface.Thus there is only a small range of angles present in the angularirradiance distribution of the projection light, and all light raysimpinge obliquely with similar angles onto the mask.

Different approaches are known in the art to modify the angularirradiance distribution of the projection light in the mask plane so asto achieve the desired illumination setting. For achieving maximumflexibility in producing different angular irradiance distribution inthe mask plane, it has been proposed to use mirror arrays that determinethe irradiance distribution in the pupil surface.

In EP 1 262 836 A1 the mirror array is realized as amicro-electromechanical system (MEMS) including more than 1000microscopic mirrors. Each of the mirrors can be tilted about twoorthogonal tilt axes. Thus radiation incident on such a mirror devicecan be reflected into almost any desired direction of a hemisphere. Acondenser lens arranged between the mirror array and a pupil surfacetranslates the reflection angles produced by the mirrors into locationsin the pupil surface. This illumination system makes it possible toilluminate the pupil surface with a plurality of spots, wherein eachspot is associated with one particular mirror and is freely movableacross the pupil surface by tilting this mirror.

Similar illumination systems using mirror arrays are known from US2006/0087634 A1, U.S. Pat. No. 7,061,582 B2 and WO 2005/026843 A2.

Although illumination systems using mirror arrays are very flexible formodifying the angular irradiance distribution, the uniformity of thespatial and angular irradiance distribution over the illuminated fieldin the mask plane is still an issue. Future generations of illuminationsystems are likely to involve a very low field dependency of thesequantities.

Some of the approaches that have been developed to reduce the fielddependency focus on the optical integrator that is usually used inillumination systems to produce a plurality of secondary light sources.The light beams emitted by the secondary light sources are superimposedby a condenser onto a mask plane or onto a field stop plane which isoptically conjugate to the mask plane. The optical integrator usuallyincludes one or more arrays of optical raster elements producing thelight beams that are associated with the secondary light sources. One ormore optical raster elements which are exclusively associated with sucha light beam form an optical channel that is independent from otheroptical channels. Since each light beam associated with an opticalchannel completely illuminates the mask or field stop plane, opticalelements located within the optical channels can be used to modify theillumination properties.

For example, U.S. Pat. No. 5,615,047 describes a plate which is arrangedin front of an optical integrator and includes a plurality of filterareas each being associated with a single optical channel of the opticalintegrator. Since the position of the filter element is opticallyconjugate to the mask or field stop plane, the transmissivitydistribution of a filter area can be selected such that a uniformspatial irradiance distribution at the mask or field stop plane isobtained.

Also U.S. Pat. No. 6,049,374 proposes to use absorptive filter elementsthat are associated with particular channels of the optical integrator.

US 2009/0021715 A1, which is assigned to the applicant of the presentapplication, describes an illumination system in which undesiredresidual field dependencies of the angular irradiance distribution areremoved. To this end optical elements such as prisms placed inindividual optical channels change certain optical properties of thelight beams associated with these optical channels.

However, there is still a desire for improvements of illuminationsystems in particular with regard to the field dependency of the angularirradiance distribution of the projection light impinging on the mask.

SUMMARY

The disclosure seeks to provide an illumination system which offersincreased flexibility with regard to the field dependency of the angularirradiance distribution at mask level. In one aspect, the disclosureprovides an illumination system including an optical integrator whichincludes an array of optical raster elements, wherein a light beam isassociated with each optical raster element. The illumination systemfurther includes a condenser which superimposes the light beamsassociated with the optical raster elements in a common field planewhich is equal to or optically conjugate to a mask plane in which a maskto be illuminated is positioned during operation of the illuminationsystem. A modulator of the illumination system is configured to modify afield dependency of an angular irradiance distribution in a field, whichis illuminated in the mask plane by the illumination system. Themodulator includes a plurality of modulator units, wherein eachmodulator unit is associated with at least one (preferably only one) ofthe light beams and is arranged at a position in front of the condensersuch that only the associated light beam impinges on the modulator unit.Each modulator unit is furthermore configured to variably redistribute,without blocking any light, a spatial and/or an angular irradiancedistribution of the associated light beam. The illumination systemfurther includes a control device which is configured to control themodulator units in such a manner that at least one modulator unitredistributes the spatial and/or the angular irradiance distribution ofan associated light beam, if the control device receives an inputcommand that the field dependency of the angular irradiance distributionin the mask plane shall be modified.

The disclosure thus departs from the conventional approach of attemptingto produce at each point of the illuminated field in the mask plane thesame well-defined angular irradiance distribution, i.e. to reduce thefield dependency of the angular irradiance distribution to very smalltolerable values. Instead, the disclosure seeks to provide anillumination system which makes it possible to enable an operator of theapparatus to quickly change the field dependency of the angularirradiance distribution in the mask plane. This makes it possible toselectively illuminate different portions of the illuminated field withdifferent angular irradiance distributions. If these distributions arespecifically adapted to the circuit pattern which is illuminated inthese portions, the pattern will be more accurately transferred to thephotoresist or another type of light sensitive surface.

However, the disclosure may also be useful for applications in which itis not desired to illuminate different portions of the mask withdifferent illumination settings. The ability to quickly modify the fielddependency of the angular irradiance distribution at mask level can thenbe used to reduce the field dependency very effectively even in cases inwhich the field dependency varies in time and thus cannot be reducedwith fixed optical elements arranged in the optical channels of theoptical integrator.

In one embodiment the modulator is configured such that a first angularirradiance distribution is produced at a first portion of theilluminated field and a second angular irradiance distribution, which isdistinct from the first angular irradiance distribution, is produced ata second portion of the illuminated field.

Particularly in apparatus of the scanner type, in which the mask ismoved along a scan direction during exposure of the photoresist, thefirst and the second portions may be formed by lines extending along thescan direction. The first portion may adjoin one end of the illuminatedfield and the second portion may adjoin an opposite end of theilluminated field. In the case of an apparatus of the scanner type theone end and the opposite end may delimit the illuminated field along adirection which is perpendicular to a scan direction.

In other embodiments the first portion is a two-dimensional area inwhich the first angular irradiance distribution is uniform, and also thesecond portion is a two-dimensional area in which the second angularirradiance distribution is uniform.

If the apparatus is of the scanner type, the illuminated field usuallyhas a long dimension along an X direction and a short dimension along aY direction which is perpendicular to the X direction and corresponds toa scan direction of the apparatus. Then the first portion may have atleast one Y coordinate, but no X coordinate, in common with the secondportion. In other words, the two portions are arranged side by sidealong the X direction, or possibly displaced along the Y direction, butdo not have a point in common which lies on a line extending parallel tothe Y direction.

In some embodiments it is even possible to vary the angular irradiancedistribution so quickly that the angular irradiance distribution changeswhile the mask is projected onto the light sensitive layer in a scanningoperation. Then the first and the second portions formed bytwo-dimensional areas may be arranged one behind the other along thescan direction Y so that the two portions may also have an X coordinatein common.

Generally the first and the second angular irradiance distributions ofthe two portions may be associated with illumination settings taken fromthe group consisting of: Conventional illumination setting, angularillumination setting, dipole illumination setting and n-poleillumination setting with n≧4.

In other embodiments each modulator unit is arranged in a raster fieldplane that is located, in a direction of light propagation, in front ofthe array of optical raster elements. Each modulator unit is configuredto variably redistribute, without blocking any light, the spatialirradiance distribution of the associated light beam in the raster fieldplane.

This exploits the fact that the raster field planes are opticallyconjugated to the common field plane, and consequently any spatialredistribution of the associated light beam in a raster field planedirectly translates into a redistributed spatial irradiance distributionin the common field plane. Since each modulator unit is associated witha particular light beam which propagates towards the common field planefrom a direction that is determined by the position of the associatedoptical raster element, the field dependency of the angular irradiancedistribution changes if a modulator unit changes the spatial irradiancedistribution that is produced by the associated light beam in the commonfield plane.

Generally the raster field planes associated with the optical rasterelements will be coplanar. However, the raster field planes may also bedisplaced along an optical axis or tilted if the optical raster elementshave different optical properties.

In some embodiments each modulator unit is configured to shift an areain the raster field plane, which is illuminated by the light beamassociated with the modulator unit, along a direction which isperpendicular to an optical axis of the illumination system. Then theilluminated field in the common field plane also shifts by an amountwhich is proportional to the shift of the area in the raster fieldplane. In an apparatus of the scanning type, the shift direction may beequal to the X direction. In this context it should be noted thatusually the raster field plane is not a plane in the mathematical sense,but is optically defined and may therefore have a certain “thickness”.Thus an oblique shift within such a “thick” field plane is stillconsidered as a shift perpendicular to the optical axis.

The shift of the illuminated area may be achieved by the modulator unitswithout changing the angular irradiance distribution of the associatedlight beams. Then the angular irradiance distribution produced by aparticular light beam in the common field plane is exclusivelydetermined by the position of the associated optical raster element, butsubstantially independent from the location of the illuminated area inthe raster field plane associated with the light beam.

Configuring a modulator unit such that it is capable of variablyredistributing the spatial irradiance distribution of the associatedlight beam in the raster field plane usually requires that there is somespace available in the raster field plane that can be used toaccommodate optical components, actuators and other mechanicalcomponents that are used for this purpose. This implies that theilluminated portions of the raster field planes have to be separated bygaps.

An optical integrator that produces raster field planes wherein theilluminated portions are separated by gaps may include, counted in adirection of light propagation, a first, a second and a third array ofoptical raster elements, wherein the raster field planes are locatedbetween the second and the third array of optical raster elements. Suchan optical integrator is described in the unpublished German patentapplication DE 10 2009 045 219 which has been filed on Sep. 30, 2009 andwhich is assigned to the applicant of the present application.

In other embodiments each modulator unit is arranged in or in closeproximity to a pupil plane that is located, in the direction of lightpropagation, behind the array of optical raster elements. Each modulatorunit is configured to variably redistribute, without blocking any light,the angular irradiance distribution of the associated light beam in thepupil plane. This exploits the fact that the angular irradiancedistribution in the pupil plane translates into a spatial irradiancedistribution in the common field plane which is Fourier related to thepupil plane.

In this context each modulator may be configured to tilt the light beamassociated with the modulator unit about a tilt axis which isperpendicular to an optical axis. This will result in a shift of thespatial irradiance distribution in the common field plane.

In the case of a scanning apparatus the tilt axis may be equal to an Ydirection which is equal to the scan direction.

Irrespective of the position of the modulator units each modulator unitmay include an optical element that is configured to change thepropagation direction of the associated light beam impinging on it.Furthermore, each modulator unit may include an actuator that is coupledto the optical element and is configured to change the position and/ororientation of the optical element in response to a control signalreceived from the control device.

In this context a parallel shift of a light beam is also considered as achange of the propagation direction.

The optical element may be a refractive optical element, in particular alens, a prism or a Fresnel prism, or a diffractive optical element.

Generally the actuator may be configured to displace the optical elementalong a direction that is inclined with respect to (and preferablyperpendicular to) an optical axis of the illumination system.

In other embodiments the actuator is configured to rotate the opticalelement around a rotational axis that is inclined with respect to (andpreferably perpendicular to) an optical axis of the illumination system.

In some embodiments the modulator is configured such that the angularirradiance distribution discontinuously varies over the illuminatedfield. This is particularly useful if masks shall be illuminated thatcontain different pattern areas each involving uniform, but differentangular irradiance distributions.

In other embodiments the modulator is configured such that the angularirradiance distribution continuously varies over at least a portion ofthe illuminated field. This may be advantageous, for example, if thedensity, the dimensions and/or the orientation of the pattern featuresare not uniform within larger pattern areas, but also vary in anapproximately continuous manner over at least a portion of theilluminated field.

In the latter case the first portion may be a first line where the firstangular irradiance distribution is uniform. The second portion may be asecond line where the second angular irradiance distribution is uniform.The modulator is then configured such that the first angular irradiancedistribution continuously transforms into the second irradiancedistribution within an area arranged between the first line and thesecond line.

For producing continuously varying irradiance distributions a modulatorunit may be used that is configured to change an irradiance distributionwithin an area in the raster field plane, which is illuminated by thelight beam associated with the modulator unit, without shifting it. Inother words, the size, geometry and position of the illuminated area inthe raster field plane is not changed by the modulator unit, but theirradiance distribution within this area does change in response to acontrol signal received from the control device.

In the case of a continuously varying angular irradiance distributioneach modulator unit may be configured to transform the irradiancedistribution from a uniform irradiance distribution into a modifiedirradiance distribution which linearly increases or decreases along areference direction. In the case of a scanning apparatus this directionmay be equal to an X direction which is perpendicular to a scandirection Y.

Subject of the disclosure is also a method of operating amicrolithographic projection exposure apparatus including the followingsteps:

-   a) providing a microlithographic projection exposure apparatus    including an illumination system and a projection objective;-   b) providing a mask to be illuminated by the illumination system;-   c) defining a first desired angular irradiance distribution and a    second desired angular irradiance distribution which is distinct    from the first angular irradiance distribution;-   d) illuminating the mask in such a way that the first angular    irradiance distribution is obtained at a first portion of the mask    and the second angular irradiance distribution is obtained at a    second portion of the mask that is distinct from the first portion.

The first and the second angular irradiance distributions may beassociated with illumination settings taken from the group consistingof: Conventional illumination setting, angular illumination setting,dipole illumination setting, n-pole illumination setting with n≧4.

The first portion may be a two-dimensional area in which the firstangular irradiance distribution is uniform. The second portion may bealso a two-dimensional area in which the second angular irradiancedistribution is uniform. Feature patterns contained in the mask may bedifferent at the first portion and the second portion.

Alternatively, the first portion may be a first line where the firstangular irradiance distribution is uniform, and the second portion maybe a second line where the second angular irradiance distribution isuniform. The first angular irradiance distribution then continuouslytransforms into the second angular irradiance distribution within anarea arranged between the first line and the second line.

The method may include the step of controlling a modulator contained inthe illumination system in such a way that the first and second angularirradiance distributions are obtained.

The method may also include the step of redistributing, without blockingany light, a spatial and/or an angular irradiance distribution of lightbeams that are associated with optical raster elements contained in theillumination system.

The angular irradiance distribution may be changed while the mask isprojected onto a light sensitive surface by the projection objective.

The present disclosure is generally applicable also to EUV illuminationsystems in which the optical raster elements are mirrors.

DEFINITIONS

The term “field plane” is used herein to denote a plane that isoptically conjugate to the mask plane.

The term “pupil plane” is used herein to denote a plane in whichmarginal rays passing through different points in the mask planeintersect.

The term “uniform” is used herein to denote a property that does notdepend on the position.

The term “light” is used herein to denote any electromagnetic radiation,in particular visible light, UV, DUV, VUV and EUV light and X-rays.

The term “light ray” is used herein to denote light whose path ofpropagation can be described by a line.

The term “light bundle” is used herein to denote a plurality of lightrays that have a common origin in a field plane.

The term “light beam” is used herein to denote light that passes througha particular lens or another optical element.

The term “orientation” is used herein to denote the angular alignment ofa body in the three-dimensional space. The orientation is usuallyindicated by a set of three angles.

The term “position” is used herein to denote the location of a referencepoint of a body in the three-dimensional space. The position is usuallyindicated by a set of three Cartesian coordinates. The orientation andthe position therefore fully describe the placement of a body in thethree-dimensional space.

The term “optical raster element” is used herein to denote any opticalelement, for example a lens, a prism or a diffractive optical element,which is arranged, together with other optical raster elements, so thata plurality of adjacent optical channels are produced or maintained.

The term “optical integrator” is used herein to denote an optical systemthat increases the product NA·a, wherein NA is the numerical apertureand a is the illuminated field area.

The term “condenser” is used herein to denote an optical element or anoptical system that establishes (at least approximately) a Fourierrelationship between two planes, for example a field plane and a pupilplane.

The term “conjugated plane” is used herein to denote planes betweenwhich an imaging relationship is established. More information relatingto the concept of conjugate planes are described in an essay E. Delanoentitled: “First-order Design and the y, y; Diagram”, Applied Optics,1963, vol. 2, no. 12, pages 1251-1256.

The term “field dependency” is used herein to denote any functionaldependency of a physical quantity from the position in a field plane.

The term “spatial irradiance distribution” is used herein to denote howthe total irradiance varies over a real or imaginary surface on whichlight impinges. Usually the spatial irradiance distribution can bedescribed by a function I_(s)(x, y), with x, y being spatial coordinatesof a point in the surface. If applied to a field plane, the spatialirradiance distribution integrates the irradiances produced by aplurality of light bundles.

The term “angular irradiance distribution” is used herein to denote howthe irradiance of a light bundle varies depending on the angles of thelight rays that constitute the light bundle. Usually the angularirradiance distribution can be described by a function I_(a)(α,β), withα, β being angular coordinates describing the directions of the lightrays. If the angular irradiance distribution has a field dependency,I_(a) will be also a function of field coordinates, i.e.I_(a)=I_(a)(α,βx,y).

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present disclosure may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic perspective view of a projection exposureapparatus in accordance with one embodiment of the present disclosure;

FIG. 2 is an enlarged perspective view of the mask to be projected bythe projection exposure apparatus shown in FIG. 1;

FIG. 3 is a meridional section through an illumination system being partof the apparatus shown in FIG. 1;

FIG. 4 is a perspective view of a mirror array contained in theillumination system shown in FIG. 3;

FIG. 5 is a perspective view of three arrays of optical raster elementscontained in the illumination system shown in FIG. 3;

FIG. 6 is a top view on an array of optical raster elements that mayalternatively be contained in the illumination system shown in FIG. 3;

FIG. 7 is a sectional view along line VII-VII of the array shown in FIG.6;

FIG. 8 is a schematic meridional section through three adjacent opticalchannels of an optical integrator contained in the illumination systemshown in FIG. 3;

FIG. 9 is a top view on a Mchannel optical integrator illustrating anirradiance distribution in a raster field plane;

FIG. 10 is a perspective view similar to FIG. 2 on a mask illustratingthe different angular irradiance distributions obtained for differentpattern areas on the mask;

FIG. 11 is a schematic meridional section similar to FIG. 8 thatadditionally shows optical components inside the modulator unit shown inFIG. 8;

FIG. 12 shows an alternative embodiment of modulator units that may beused in the embodiment shown in FIG. 8;

FIG. 13 is a schematic meridional section through the optical integratorof the illumination system shown in FIG. 3 illustrating the focallengths of the optical raster elements contained therein;

FIG. 14 is a meridional section through an illumination system accordingto another embodiment in which modulator units are arranged in a pupilplane of the illumination system;

FIG. 15 is a schematic meridional section through three adjacent opticalchannels of the optical integrator shown in FIG. 14 in a representationsimilar to FIG. 8;

FIG. 16 is a cut-out from FIG. 15 showing optical elements within themodulator units;

FIG. 17 is a cross-section in an XZ plane through a Fresnel prism thatmay alternatively be used as optical element in the modulator unitsshown in FIG. 16;

FIG. 18 is a perspective view of a still further embodiment of anoptical element contained in a modulator unit including two wedges;

FIG. 19 is a schematic illustration of the angular irradiancedistribution that discontinuously varies along an X direction of theilluminated field;

FIG. 20 is an illustration of the irradiance distribution in the rasterfield planes which produces the angular irradiance distribution shown inFIG. 19;

FIG. 21 is a schematic illustration of an angular irradiancedistribution that continuously varies along an X direction of theilluminated field;

FIG. 22 is an illustration of the spatial irradiance distributions thatare produced by the optical channels in the common field plane so as toproduce the varying angular irradiance distribution illustrated in FIG.21,

FIG. 23 is a schematic meridional section through three adjacent opticalchannels of an illumination system that produces a continuously varyingangular irradiance distribution in the mask plane;

FIG. 24 is a cut-out from FIG. 23 that schematically illustrates how atop-hat irradiance distribution is transformed by the modulator unitsinto different linearly decreasing or increasing irradiancedistributions;

FIG. 25 is a graph indicating two optical surfaces of an optical membercontained in the modulator units shown in FIGS. 23 and 24;

FIG. 26 is a graph showing the shape of the optical member in an XZplane;

FIG. 27 is a perspective view of the optical element including two ofthe optical members as shown in FIG. 26;

FIGS. 28 to 30 are front views, along the Z direction, of the opticalelement shown in FIG. 27 which is arranged in different X positions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I General Construction ofProjection Exposure Apparatus

FIG. 1 is a perspective and highly simplified view of a projectionexposure apparatus 10 in accordance with the present disclosure. Theapparatus 10 includes an illumination system 12 which produces aprojection light beam. The latter illuminates a field 14 on a mask 16containing a pattern 18 formed by a plurality of small features 19 thatare schematically indicated in FIG. 1 as thin lines. In this embodimentthe illuminated field 14 has the shape of a ring segment which does notcontain an optical axis OA of the apparatus. However, other shapes ofthe illuminated field 14, for example rectangles, are contemplated aswell.

A projection objective 20 images the pattern 18 within the illuminatedfield 14 onto a light sensitive layer 22, for example a photoresist,which is supported by a substrate 24. The substrate 24, which may beformed by a silicon wafer, is arranged on a wafer stage (not shown) suchthat a top surface of the light sensitive layer 22 is precisely locatedin an image plane of the projection objective 20. The mask 16 ispositioned via a mask stage (not shown) in an object plane of theprojection objective 20. Since the latter has a magnification β with|β|<1, a minified image 18′ of the pattern 18 within the illuminatedfield 14 is projected onto the light sensitive layer 22.

During the projection the mask 16 and the substrate 24 move along a scandirection which corresponds to the Y direction indicated in FIG. 1. Theilluminated field 14 then scans over the mask 16 so that patterned areaslarger than the illuminated field 14 can be continuously imaged. Theratio between the velocities of the substrate 24 and the mask 16 isequal to the magnification β of the projection objective 20. If theprojection objective 20 inverts the image (β<0), the mask 16 and thesubstrate 24 move in opposite directions, as this is indicated in FIG. 1by arrows A1 and A2. However, the present disclosure may also be used instepper tools in which the mask 16 and the substrate 24 do not moveduring projection of the mask.

II Multiple Illumination Setting

FIG. 2 is an enlarged perspective view of the mask 16. The pattern 18 onthis mask includes three first identical pattern areas 181 a, 181 b, 181c which are arranged one behind the other along the Y direction. For thesake of simplicity it is assumed that the features 19 of the firstpattern areas 181 a, 181 b, 181 c are straight lines extending along theY direction. The pattern 18 further includes three identical secondpattern areas 182 a, 182 b, 182 c which are also arranged one behind theother along the Y direction, but laterally displaced from the firstpattern areas 181 a, 181 b, 181 c so that the first pattern areas 181 a,181 b, 181 c and the second pattern areas 182 a, 182 b, 182 c have nocommon X coordinate. It is assumed that the second pattern areas 182 a,182 b, 182 c contain features 19 extending along the X direction andfeatures 19 extending along the Y direction.

The mask 16 shown in FIG. 2 is assumed to be used in a manufacturingstep in which two different dies are exposed simultaneously and will besubjected to the same subsequent manufacturing steps such as etching.The dies are small enough so that they can be positioned next to eachother within the image field of the projection objective 20 having awidth w along the X direction, as it is shown in FIG. 1. Within onecomplete scanning cycle three dies of a first type associated with thefirst pattern areas 181 a, 181 b, 181 c and three dies of a second typeassociated with the second pattern areas 182 a, 182 b, 182 c can beexposed. Then the scanning direction is either reversed or the mask 16is returned to its original position without any illumination, and afurther scanning cycle is performed. In this way two rows of differentdies can be exposed simultaneously on the substrate 24.

Generally different patterns require different angular irradiancedistributions at mask level if maximum image quality is desired. In thisembodiment it is assumed that the features 19 extending along the Ydirection are best imaged on the light sensitive layer 22 with an Xdipole illumination setting. In FIG. 2 the pupil 26 associated with alight bundle that converges towards a field point located in one of thefirst pattern areas is indicated by a broken circle. In the pupil 26 twopoles 27, which are spaced apart along the X direction, representdirections from which light propagates towards the field point. Sincethe patterns are assumed to be uniform over the first pattern areas 181a, 181 b, 181 c, thus this X dipole illumination setting is produced ateach field point in the first pattern areas 181 a, 181 b, 181 c.

The second pattern areas 182 a, 182 b, 182 c, which are associated withthe second type of die, contain features extending along the X directionand features 19 extending along the Y direction. For these features 19it is assumed that an annular illumination setting results in the bestimage quality. FIG. 2 indicates an annulus 28 which is illuminated inthe pupil 26 associated with a light bundle that converges towards afield point in one of the second pattern areas 182 a, 182 b, 182 c.Again, this annular illumination setting shall be produced at each fieldpoint in the second pattern areas 182 a, 182 b, 182 c.

This implies that the illumination system 12 is capable of producing twodifferent illumination settings simultaneously and side by side withinthe illuminated field 14. In the following the structure of theillumination system 12 which is capable of performing this task will bedescribed in more detail with reference to FIGS. 3 to 13.

III General Construction of Illumination System

FIG. 3 is a meridional section through the illumination system 12 shownin FIG. 1. For the sake of clarity, the illustration of FIG. 3 isconsiderably simplified and not to scale. This particularly implies thatdifferent optical units are represented by one or very few opticalelements only. In reality, these units may include significantly morelenses and other optical elements.

The illumination system 12 includes a housing 29 and a light source 30that is, in the embodiment shown, realized as an excimer laser. Thelight source 30 emits projection light having a wavelength of about 193nm. Other types of light sources 30 and other wavelengths, for example248 nm or 157 nm, are also contemplated.

In the embodiment shown, the projection light emitted by the lightsource 30 enters a beam expansion unit 32 in which the light beam isexpanded. The beam expansion unit 32 may include several lenses or maybe realized as a mirror arrangement, for example. The projection lightemerges from the beam expansion unit 32 as an almost collimatedprojection light beam 34.

The projection light beam 34 then enters a pupil defining unit 36 thatis used to produce variable spatial irradiance distributions in asubsequent pupil plane. To this end the pupil defining unit 36 includesan array 38 of microscopic mirrors 40 that can be tilted individuallyabout two orthogonal axes with the help of actuators. FIG. 4 is aperspective view of the array 38 illustrating how two parallel lightbeams 42, 44 are reflected into different directions depending on thetilting angles of the mirrors 40 on which the light beams 42, 44impinge. In FIGS. 3 and 4 the array 38 includes only 66 mirrors 40; inreality the array 38 may include several hundreds or even severalthousands mirrors 40.

The pupil defining unit 36 further includes a prism 46 having a firstplane surface 48 a and a second plane surface 48 b that are bothinclined with respect to an optical axis OA of the illumination system12. At these inclined surfaces 48 a, 48 b impinging light is reflectedby total internal reflection. The first surface 48 a reflects theimpinging light towards the mirrors 40 of the mirror array 38, and thesecond surface 48 b directs the light reflected from the mirrors 40towards an exit surface 49 of the prism 46. The angular irradiancedistribution of the light emerging from the exit surface 49 can thus bevaried by individually tilting the mirrors 40 of the array 38. Moredetails with regard to the pupil defining unit 38 can be gleaned from US2009/0116093 A1.

The angular irradiance distribution produced by the pupil defining unit36 is transformed into a spatial irradiance distribution with the helpof a first condenser 50 which directs the impinging light towards anoptical integrator 52. In this embodiment the optical integrator 52includes a first array 54 a, a second array 54 b and a third array 54 cof optical raster elements 56.

FIG. 5 is a perspective view of the three arrays 54 a, 54 b, 54 c. Eacharray includes, on a front and a rear side of a support plate, asub-array of optical raster elements 56 which are formed by parallelcylinder lenses that extend along the X or Y direction. The use ofcylinder lenses is often advantageous particularly in those cases inwhich the refractive power of the optical raster elements 56 shall bedifferent along the X and the Y direction.

FIGS. 6 and 7 show the array 54 a according to an alternative embodimentin a top view and a sectional view along line VII-VII, respectively.Here the optical raster elements 56 are formed by plano-convex lenseshaving a square contour. The other arrays 54 b, 54 c differ from thearray 54 a only with regard to the curvature of the convex surface ofthe optical raster elements 56.

Referring again to FIG. 3, the optical raster elements 56 of the first,second and third array 54 a, 54 b and 54 c, respectively, are arrangedone behind the other in such a way that one optical raster element 56 ofeach array is exactly associated with one optical raster element 56 ofthe other two arrays. The three optical raster elements which areassociated with each other are aligned along a common axis and define anoptical channel. Within the optical integrator 52 a light beam whichpropagates in one optical channel does not cross or superimpose withlight beams propagating in other optical channels. In other words, theoptical channels which are associated with the optical raster elements56 are optically isolated from each other.

Between the second array 54 b and the third array 54 c a raster fieldplane 58 is located in which modulator units 60 of a modulator 62 arearranged. The modulator units 60 are connected via control lines 64 to acontrol device 66 which is, in turn, connected to a central apparatuscontrol 68 which controls the overall operation of the projectionexposure apparatus 10.

In this embodiment a pupil plane 70 of the illumination system 12 islocated behind the third array 54 c (it may also be arranged in front ofit). A second condenser 72 establishes a Fourier relationship betweenthe pupil plane 70 and a field stop plane 71 in which an adjustablefield stop 74 is arranged. The field stop plane 71 is opticallyconjugated to the raster field plane 58 in which the modulator units 60are arranged.

This means that an area in the raster field plane 58 within an opticalchannel is imaged on the field stop plane 71 by the associated opticalraster element 56 of the third array 54 c and the second condenser 72.The images of the illuminated areas within the optical channelssuperimpose in the field stop plane 71, and this results in a veryhomogenous illumination of the field stop plane 71. This process isoften described by identifying the illuminated areas in the opticalchannels with secondary light sources that commonly illuminate the fieldstop plane 71.

The field stop plane 71 is imaged by a field stop objective 76 onto amask plane 78 in which the mask 16 is arranged with the help of a maskstage (not shown). The adjustable field stop 74 is also imaged on themask plane 78 and defines at least the short lateral sides of theilluminated field 14 extending along the scan direction Y.

IV Modulator

The function of the modulator 62 will be explained in the following withreference to FIG. 8 which is a schematic meridional section throughthree adjacent optical channels I, II and III formed in the opticalintegrator 52. The projection light impinging on the optical integrator52 has a low divergence. For the sake of simplicity this divergence isneglected in this discussion so that the light impinging on the firstarray 54 a of the optical raster element 56 is assumed to be collimated.It is further assumed that the three optical raster elements 56 of theoptical integrator 52 are uniformly illuminated, as it is indicated inFIG. 8 by arrows A3. For the sake of simplicity the optical rasterelements 56 which are formed where orthogonal cylinder lenses intersectare represented as biconvex lenses.

The optical raster elements 56 of the first two arrays 54 a, 54 b havethe effect that the width of the light beams associated with theindividual optical channels I, II and III is reduced in the X direction.A reduction may also take place along the Y direction, but possibly witha different reduction factor. The areas illuminated on the modulatorunits 60 have a rectangular shape and are, at least along the Xdirection, separated by gaps that are represented in FIG. 8 by the blankspace between adjacent pairs of arrows A4.

The modulator units 60 have the effect that these illuminated areas inthe raster field plane 58 are laterally shifted along the X direction.This lateral shift is represented in FIG. 8 by pairs of arrows A5 forthe optical channels II and III; in the upper optical channel I themodulator unit 60 is in a neutral state so that the irradiancedistribution is not shifted.

The optical raster element 56 of the third array 54 c and the secondcondenser 72 image the irradiance distributions in the raster fieldplanes 58 on the field stop plane 71, as it has already been mentionedabove. The irradiance distribution in the field stop plane 71 producedexclusively by the upper optical channel I is indicated in FIG. 8 by arectangle drawn in solid lines. This irradiance distribution is centeredin the field stop plane 71, since the modulator unit 60 has not shiftedthe irradiance distribution at its entrance side.

However, the irradiance distributions in the field stop plane 71produced by the middle and the lower optical channels II and III, whichare indicated in FIG. 8 with broken and dotted lines, respectively, arenow laterally shifted along the X direction. This is simply a result ofthe optical conjugation between the raster field plane 58 and the fieldstop plane 71 in each optical channel.

The angular irradiance distribution, which is produced in the field stopplane 71 by each of the optical channels I, II and III, depends on theposition of the optical channel in the pupil plane 70. The greater thedistance between the optical axis of the second condenser 72 and theposition of an optical channel is, the larger will be the illuminationangle produced by the optical channel. Thus the three optical channelsI, II and III are able to produce different illuminated fields havingdifferent angular irradiance distributions.

This will be explained in the following in more detail with reference toFIGS. 9 and 10. FIG. 9 is a top view of the raster field plane 58 of anoptical integrator 52 in which only 3×3 optical channels are provided.The dark areas in FIG. 9 indicate areas in the raster field plane 58which are illuminated by projection light. It can be seen in FIG. 9 thatfive areas 80 are shifted along the −X and two areas 81 are shiftedalong the +X direction as a result of the operation of the modulatorunits 60 associated with the respective optical channels. Two areas 82are not illuminated, i.e. the pupil defining unit 36 does not direct anylight towards the optical raster elements 56 associated with these areas82.

As it has been explained above, the lateral shift of the areas 80illustrated in FIG. 9 has the effect that also the illuminated area inthe field stop plane 71, and consequently the mask plane 78, is shifted.By suitably selecting the dimensions of the areas 80 it can be achievedthat either the left half or the right half of the field 14 on the mask16 is illuminated by the respective optical channel.

FIG. 10 is a perspective view on the mask 16 and illustrates theillumination conditions for this simplified example. It can be seen thaton one half of the illuminated field 14 an angular irradiancedistribution is obtained which resembles a C-quad illumination settingincluding five poles, namely four outer poles 83 a and a central pole 83b. These five poles correspond to the five illuminated areas 80 in shownin FIG. 9.

On the other half of the field 14 an angular irradiance distribution isobtained which resembles a Y dipole illumination setting including twopoles 84. These two poles 84 correspond to the two illuminated areas 81shown in FIG. 9.

From the foregoing it should have become clear that almost any arbitraryillumination setting can be produced on the two halves of theilluminated field 14 if the number of optical channels is sufficientlyhigh, provided the pupil defining unit 36 is also able to produce adesired irradiance distribution on the optical integrator 52. In thefollowing two different embodiments of the modulator units 60 aredescribed with reference to FIGS. 11 and 12.

In the embodiment shown in FIG. 11 the modulator units 60 of themodulator 62 each include two cylinder lenses 86, 88 that can beindividually displaced along the X direction, as it is indicated bydouble arrows in FIG. 11. By decentering the cylinder lenses 86, 88 fromthe optical axis of the respective optical channel, the light beamsassociated with the optical channels are laterally displaced. Thisexploits the fact that a decentered lens has the same effect as acentered lens plus a triangular prism. For displacing the cylinderlenses 86, 88 actuators indicated at 90, 92 are coupled to the cylinderlenses 86, 88. The actuators 90, 92 change the position of the lenses86, 88 in response to control signals received from the control device66.

FIG. 12 shows another embodiment of a modulator 62 in a meridionalsection. In this embodiment each modulator unit 60 includes a prism 94having the shape of a parallelepiped. Each prism 94 has two pairs ofplanar rectangular surfaces and one pair of planar surfaces having acontour of a parallelogram. With the help of actuators indicated at 96the prisms 94 can be rotated around a rotational axis 98.

In the rotational position shown for the upper optical channel I theprism 94 is in a neutral state in which the light beam passes throughtwo planar surfaces at normal incidence. In the rotational positionsshown for the middle and lower optical channel II and III the lightbeams pass through two inclined planar surface so that the light beamsare laterally shifted.

FIG. 13 is a schematic meridional section through one optical channel ofthe optical integrator 52. In this illustration the ray traces of acentral light bundle 100 and a marginal light bundle 102 are shown insolid and broken lines, respectively. The focal lengths of the opticalraster elements 56 of the three arrays 54 a, 54 b and 54 c are indicatedas f_(a), f_(b) and f_(c). The hatched area 104 in the raster fieldplane 58 indicates a volume through which no projection light passes andwhich is thus available for accommodating components such as actuators,support structures or axes of the modulator units 60.

The irradiance distribution on the optical raster element 56 of thefirst array 54 a having a diameter d is imaged at a reduced scale d′/donto the raster field plane 58 where the diameter is d′. As can be seenfrom the gap between adjacent focal planes, the optical raster elements56 are positioned in a slightly defocused manner. This enablesadjustments to correct telecentricity errors, for example. More detailsregarding the optical integrator 52 can be gleaned from the abovementioned unpublished German patent application DE 10 2009 045 219 whichhas been filed on Sep. 30, 2009 and which is assigned to the applicantof the present application.

V Alternative Embodiments

FIG. 14 is a schematic meridional section similar to FIG. 3 through anillumination system 112 according to another embodiment. The opticalintegrator 152 includes in this embodiment only two arrays 154 a, 154 bof optical raster elements 156. However, the main difference to theillumination system 12 shown in FIG. 3 is that the modulator 162including the modulator units 160 is not arranged in the raster fieldplane 58, but in the pupil plane 70 immediately behind the last array ofoptical raster elements 156, between the second array 154 b and thesecond condenser 72. Furthermore, the modulator units 160 are configuredto variably redistribute, without blocking any light, not the spatialirradiance distribution, but the angular irradiance distribution of theassociated light beams in the pupil plane 70.

This will be explained in more detail with reference to FIG. 15 whichshows, in a representation similar to FIG. 8, three adjacent opticalchannels I, II and II of the optical integrator 152.

The modulator units 160 are arranged at a position behind the secondarray 154 b where the light beams associated with the optical channelsI, II and II of the optical integrator 152 not yet superimpose. Thus thelight impinging on each of the modulator units 160 is associated withonly one of the optical channels I, II and III. As it has been mentionedabove, the modulator units 160 modify the angular irradiancedistribution of the associated light beams, which becomes clear fromcomparing the arrows A7 with the arrows A6 that represent light rays ofthe associated light beams behind and in front of the modulator units160, respectively. The second condenser 72 translates the differentangular irradiance distributions into different spatial irradiancedistributions in the field stop plane 71.

In the upper optical channel I the modulator unit 160 is in an operatingstate in which the divergence of the light beam is increased.Consequently the spatial irradiance distribution illustrated in thefield stop plane 71 by a solid line 106, has its maximum dimension alongthe X direction.

In the middle optical channel II the modulator unit 160 is in anoperating state in which the divergence is not increased, but the lightbeam associated with this optical channel is tilted in the −X direction.This results in a spatial irradiance distribution which is representedin FIG. 15 by a broken line 108. This spatial irradiance distributionhas a width along the X direction which is one half of the maximum widthproduced by the upper optical channel I, an irradiance level which istwice as high as the irradiance level produced by the upper opticalchannel II.

In the lower optical channel III the modulator unit 160 is in anoperating state in which the light beam associated with this opticalchannel is tilted towards the +X direction. This results in the spatialirradiance distribution that is represented in FIG. 15 by a broken line110.

Thus it is again possible, here by tilting the light beams associatedwith the modulator units 160 about a tilt axis which is parallel to theY axis and thus perpendicular to the optical axis OA of the illuminationsystem 112, to illuminate with a particular optical channel differentportions of the field stop plane 71 and thus of the mask 16. If themodulator unit 160 of the upper optical channel was configured such thatin a neutral operating state the divergence is not increased, themodulator 162 would have the same effect as the modulator 62 shown inFIG. 8.

FIG. 16 is a schematic meridional section through the optical integrator152 and the modulator 160 for the three adjacent optical channels I, IIand III. Each modulator unit 160 includes a triangular prism 113 and anactuator (not shown) which is configured to displace the prism 110 alongthe +X or −X direction in response to a control signal received from thecontrol device 66. In the neutral position of the prism 113, which isshown for the modulator unit 160 associated with the upper opticalchannel I, the divergence is increased, but the light beam is not tiltedin its entirety. If the actuators are operated and the prism 113 isdisplaced laterally along the −X or +X direction, as it is shown for thetwo modulator units 160 associated with the middle optical channel IIand the lower optical channel III, the light beams associated with theseoptical channels are tilted about the Y direction, as it has beendescribed above with reference to FIG. 15.

If the prism 113 is in a position between the centered position shownfor the upper optical channel I and one of the end positions shown forthe middle and lower optical channels II and III, a stepped irradiancedistribution with two non-zero irradiance levels will be obtained in thefield stop plane 71. The ratio between these two levels depends on the Xposition of the prism 113. Thus each optical channel can directarbitrary fractions of light towards the two halves of the illuminatedfield in the field stop plane 71.

Also in this embodiment it is advantageous to have free space availablebetween the optical channels I, II and III for accommodating theactuators that displace the prisms 113. This can be achieved byappropriately designing the optical integrator 152.

FIG. 17b is a cross section in an XZ plane through a prism 113′according to an alternative embodiment. The prism 113′ of thisembodiment is a “fresnelized” equivalent of the triangular prism 113shown in FIG. 16. The Fresnel prism 113′ thus does not have a crosssection which is substantially triangular, but has a saw-tooth likestepped contour, as it is shown in FIG. 17. The Fresnel prism 113′ mayhave advantages with regard to aberrations that may otherwise beproduced by the triangular prism 113 shown in FIG. 16.

If the prisms 113 or 113′ cannot be displaced along the X direction, forexample because there is no available space for enabling the shiftingmovements of the prisms 113, 113′ or for accommodating the actuators andsupport structures or other mechanical components, the prisms may bereplaced by another refractive optical element that is displaced alongthe Y direction to modify the angular irradiance distribution of thelight beams associated therewith.

FIG. 18 is a perspective view of such a refractive optical element whichis denoted in its entirety by 116. The optical element 116 includes tworefractive wedges 118, 120 which are arranged side by side, wherein onewedge 118 is in a position that is obtained from the position of theother wedge 120 by rotating the wedge 118 by 180° about a rotationalaxis which is parallel to the Z direction. If the refractive opticalelement 116 is used in the modulator units 160 shown in FIG. 16 so thatthey can be displaced along the Y direction, it is possible, providedthe dimensions are suitably determined, that either the wedge 118 or thewedge 120 is completely exposed to the light beam associated with therespective optical channel. Then the same effect is achieved as it isshown for the middle optical channel II and the lower optical channelIII in FIG. 16. If the refractive optical element 116 is in a centeredposition in which one half of the light beam passes through the wedge118 and the other half through the wedge 120, substantially the sameeffect as shown for the upper optical channel I in FIG. 16 is achieved.

VI Irradiance Management

In the foregoing only little attention has been given to the issue howthe available amount of projection light is distributed over the variousoptical channels so that the desired irradiance and angular lightdistribution in the mask plane is obtained.

In the following it will be described how the irradiance management isperformed if the illumination settings shown in FIG. 2 for the differentpattern areas 181 a, 181 b, 181 c on the one hand and 182 a, 182 b, 182c on the other hand shall be achieved.

For the sake of simplicity it will be assumed that the available numberof optical channels in the optical integrator is 6×6. FIG. 19 shows forthis case schematically how the irradiance distribution in the pupil 26associated with individual field points (referred to in the following aspupil irradiance distribution) has to vary along the X direction overthe illuminated field 14. At the left half of the illuminated field 14the illumination setting 122 shall be annular, and on the right half ofthe illuminated field 14 an X dipole illumination setting 124 shall beset. These two different illumination settings are approximated in therepresentation of FIG. 19 only roughly as a result of the restrictednumber of available optical channels.

It is further assumed that in the case of the annular illuminationsetting 122 the total area illuminated in the pupil is twice as large asin the case of the X dipole setting. Since the points on the mask 16 aresupposed to receive the same amount of light irrespective whether theyare located on the left or the right half of the illuminated field 14,the irradiance at the illuminated pupil areas is half as high for theannular illumination setting if compared to the X dipole illuminationsetting. This is illustrated in FIG. 19 by areas 126, 127 that areassociated with different optical channels of the optical integrator 152and have a different blackening.

These pupil irradiance distributions, which are different for fieldpoints in the left and the right half of the illuminated field 14, haveto be produced by the optical integrator 152 and the modulator 162. FIG.20 illustrates the irradiance distribution which is used to this end inthe pupil plane 70 of the illumination system 112. It can be seen thatthere are four different irradiance levels, namely a zero level 128(white), a one third irradiance level 130 (light grey), a two thirdirradiance level 132 (dark grey) and a full irradiance level 134(black).

The highest irradiance level 134 is used for those optical channels thathave to direct light to both halves of the illuminated field 14. Morespecifically, these optical channels have to direct one third of theavailable light to the left half of the illuminated field 14, in whichthe annular illumination setting shall be produced, and the remainingtwo thirds of the available light has to be directed to the right halfof the illuminated field, in which the X dipole illumination settingshall be produced. Such a stepped irradiance distribution in the fieldstop plane 71 or the mask plane 78 can be obtained, for example, with aprism 113 or 113′ being in a position between the centered positionshown for the upper optical channel I in FIGS. 15 and 16 and the endpositions shown for the middle optical channel II and the lower opticalchannel III.

The two third irradiance level 132 is used for those optical channelsthat exclusively direct their light to the right half of the illuminatedfield 14 so as to obtain the X dipole illumination setting 124. As ithas been mentioned above, there the irradiance has to be twice as largeas for the areas that direct their light exclusively to the left half ofthe illuminated field 14 where the annular illumination setting 122shall be obtained. At these areas the one third irradiance level 130 isused.

The zero irradiance level 128 is used at those areas in the pupil plane70 that shall direct no light at all to the illuminated field 14.

The four different irradiance levels 128, 130, 134, 132 can be easilyachieved with the help of the mirror array 38 of the pupil defining unit36. If we assume, for the sake of simplicity, that the array 38 includes(only) 36 mirrors that each produce the same irradiance, three mirrors40 may direct the projection light to each optical channel at which thefull irradiance level 134 is used, two mirrors 40 may direct theprojection light to each optical channel at which the two thirdsirradiance level 132 is used, and one mirror 40 may direct theprojection light to each optical channel at which the one thirdirradiance level 130 is used. From the total of 36 mirrors four mirrorswould remain that do not direct any light to the optical integrator 52at all.

Often, however, the mirrors do not produce the same irradiance, as ithas been assumed above, but quite different (albeit known) irradiances.Then the mirrors 40 producing the highest irradiances may be controlledin such a way that they direct projection light to those areas where thefull irradiance level 134 is used. Mirrors 40 producing about two thirdsof the full level 134 are controlled such that they direct projectionlight to areas where the two third irradiance level 132 is used, and soon.

VII Continuous Variation of Angular Irradiance Distribution

In the foregoing it has been assumed that there are two portions on themask 16 which are located side by side along the X direction that shallbe illuminated with different angular irradiance distributions. However,it may also be envisaged to illuminate the mask 16 in such a way thatthe angular irradiance distribution varies continuously, and inparticular along a direction which is perpendicular to the scandirection Y.

FIG. 21 illustrates, in a representation similar to FIG. 19, how thepupil irradiance distribution may vary within the illuminated field 14along the X direction. At the left end of the illuminated field 14 anannular illumination setting 122 is produced. At the right end of theilluminated field 14 an X dipole illumination setting 124 is produced.The different grey shadings in the pupils between these opposite endsindicate how the angular irradiance distribution continuously variesbetween the two ends of the illuminated field 14.

Thus each line extending along the Y direction forms a portion in whichthe angular irradiance distribution is uniform, but this distributioncontinuously varies between the two special distributions at theopposite ends of the illuminated field 14 that are associated with theangular illumination setting 122 and the X dipole illumination setting124.

FIG. 22 illustrates for this scenario the irradiance distributions thathave to be produced in the field stop plane 71 by each of a totality of66 optical channels. By comparing FIGS. 21 and 22 it can be seen thatsome optical channels 136 have to produce an irradiance distributionwhich linearly decreases from a half maximum value at the left end ofthe illuminated field to zero at the right end of the illuminated field14. Other optical channels 138 have to produce an irradiancedistribution in the field stop plane 71 which linearly increases fromzero to a maximum irradiance level. Still other optical channels 140have to produce an irradiance distribution in the field stop plane 71which linearly increases from a half maximum value at the left end ofthe illuminated field 14 to a maximum value at the right end of theilluminated field 14.

Such irradiance distributions can be produced with a modulator 262 thatreplaces the modulator 60 in the embodiment shown in FIG. 3. Thefunction of the modulator 262 will now be explained with reference toFIG. 23 which is a meridional section through three adjacent opticalchannels I, II and III of the optical integrator 52 and modulator units260 associated with the optical channels I, II and III. Other than themodulator units 60 shown in FIG. 8, the modulator units 262 shown inFIG. 23 are configured to change the irradiance distribution within theraster field plane 58, which is illuminated by the light beam associatedwith the modulator units 260, without shifting the irradiancedistribution. In other words, the light is redistributed within theilluminated area, but the position of the area as such is not changed.In FIG. 23 this is represented by arrows A8 having a different thicknessthat indicates the irradiance at this X coordinate.

FIG. 24 illustrates this redistribution of the spatial irradiancedistribution schematically for the three optical channels I, II and IIIshown in FIG. 23. Again it is assumed that the irradiance distributionat the entrance side of the modulator units 260 is uniform (i.e. a tophat distribution), as it is denoted by rectangles 142 in FIG. 23. Themodulator units 260 transform this rectangular irradiance distribution142 into the linearly decreasing or increasing distributions 144 a, 144b and 144 c shown on the right hand side of FIG. 24.

Referring again to FIG. 23, this redistribution of the spatial lightdistribution performed by the modulator units 260 produces the desiredirradiance distributions 136, 138, 140 in the field stop plane 71,because the raster field plane 58 is optically conjugated to the fieldstop plane 71 by the optical raster elements 56 of the third array 54 cand the second condenser 72.

The redistributions of a uniform spatial irradiance distribution 142into various linearly increasing or decreasing spatial irradiancedistributions, as it is schematically shown in FIG. 23, can be producedwith the help of refractive optical elements that are contained in themodulator units 260. In the following it will be exemplarily describedhow a refractive optical element is shaped so that a uniform irradiancedistribution 142 is transformed into an irradiance distribution 144 bwhich linearly increases from zero to a maximum value.

FIG. 25 shows, as a starting point, a refractive optical member 146having a front surface 148 and a rear surface 149. The shape of thefront surface is assumed to be given by an equation w₁(x), and the shapeof the rear surface 149 by an equation w₂(x′). It is further assumedthat a light ray which enters the front surface 148 at a position x isrefracted at the front surface 148 and leaves the rear surface 149 at aposition x′.

The total light energy in a small volume element is preserved, i.e.I(x)dx=I′(x′)dx′  (1)

Assuming that the irradiance distribution 142 at the front surface 148is uniform, the irradiance distribution I′(x′) at the rear surface 149will be

$\begin{matrix}{{I^{\prime}(x)}^{\prime} \propto \frac{\mathbb{d}x}{\mathbb{d}x^{\prime}}} & (2)\end{matrix}$

If the irradiance distribution shall increase linearly, the equation (3)

$\begin{matrix}{\frac{\mathbb{d}x}{\mathbb{d}x^{\prime}} = \frac{2\; x^{\prime}}{L}} & (3)\end{matrix}$has to be fulfilled, with L being the width of the illuminated area inthe raster field plane 58.

Then the equations which have to be solved are (in paraxialapproximation)

$\begin{matrix}{{\frac{\mathbb{d}w_{1}}{\mathbb{d}x} = {\frac{n}{n - 1} \cdot \frac{x - x^{\prime}}{w_{2} - w_{1}}}}{\frac{\mathbb{d}w_{2}}{\mathbb{d}x^{\prime}} = {\frac{n}{n - 1} \cdot \frac{x - x^{\prime}}{w_{2} - w_{1}}}}{\frac{\mathbb{d}x}{\mathbb{d}x^{\prime}} = \frac{2\; x^{\prime}}{L}}} & (4)\end{matrix}$

This set of equations may be understood simply: A ray which enters therefractive optical member 146 at a position x leaves the rear surface149 at the position x′. The thickness between the front surface 148 andthe rear surface 149 is given by w₂(x)−w₁(x) which results in adeviation angle (x′−x)/(w₂−w₁). The inclination of the front surface 148is then defined by the right hand side of the equation for dw₁/dx. Therear surface 149 has the task of changing the direction of the light rayagain to its original direction which it had when it entered the frontsurface 148.

With the auxiliary function

$\begin{matrix}{{a(s)} = {\tan\left( {{\sin^{- 1}\left( \frac{s}{n\sqrt{1 + s^{2}}} \right)} - {\tan^{- 1}s}} \right)}} & (5)\end{matrix}$the equation (4) can be rewritten as

$\begin{matrix}{{\frac{\mathbb{d}w_{1}}{\mathbb{d}x} = {a^{- 1}\left( \frac{x - x^{\prime}}{w_{2} - w_{1}} \right)}}{\frac{\mathbb{d}w_{2}}{\mathbb{d}x} = {a^{- 1}\left( \frac{x - x^{\prime}}{w_{2} - w_{1}} \right)}}{\frac{\mathbb{d}x}{\mathbb{d}x^{\prime}} = \frac{2\; x^{\prime}}{L}}} & (6)\end{matrix}$

Numerically solving the equations (6) yields a refractive optical member146 which has, in an XZ plane, the shape which is shown in FIG. 26.

FIG. 27 shows an optical element 150 which is contained in eachmodulator unit 260 and can be displaced along the Y direction with thehelp of an actuator (not shown). The optical element 150 consists of afirst refractive optical member 146 a and a second optical member 146 bthat both have the shape as shown in FIG. 26. The optical element 150 isassembled from the two optical members 146 a, 146 b with the opticalmember 146 b being rotated by 180° around the Z axis.

The first optical member 146 a transforms a uniform irradiancedistribution into an irradiance distribution which decreases linearlyfrom a maximum value to 0 along the X direction, as it is indicated byan irradiance distribution 144 a on the right hand side if FIG. 27. Thesecond optical member 146 b redistributes the light such that theirradiance distribution obtained at its rear surface 149 linearlyincreases from 0 to a maximum value, as it is indicated by a broken line146 b. If the optical element 150 is displaced along the Y direction sothat light impinges on both optical members 146 a, 146 b, the twolinearly increasing and linearly decreasing irradiance distributions 144a, 144 b are superimposed so that also linearly increasing distributions144 c between a first non-zero level and a second non-zero level can beproduced, as it is shown in FIG. 24.

This is illustrated in FIGS. 28 to 30 which show the front surfaces 140of the optical members 146 a, 146 b and an actuator 152 in threedifferent operating states. The actuator 152 is configured to displacethe optical element 150 along the Y direction.

In a first operating state shown in FIG. 28 an area 154, which isilluminated by the optical raster elements 56 of the first and secondarray 54 a, 54 b of one of the optical channels, lies completely withinthe front surface 140 of the first optical member 146 a. As a result,the irradiance distribution 144 a is produced at the rear surface 149 ofthe first refractive optical member 146 a. Due to the opticalconjugation this optical channel then produces the irradiancedistribution 144 a also at the mask plane 78.

FIG. 29 shows the optical element 150 in a second operating state inwhich the area 154 lies completely within the front surface 140 of thesecond refractive optical member 146 b. Then the optical channelproduces an irradiance distribution 144 a which has the opposite fielddependency, i.e. it linearly increases along the −X direction.

In the third operating state shown in FIG. 30 the optical element 150has been moved along the Y direction such that a larger portion of theilluminated area 154 lies within the front surface 140 of the firstoptical member 146 a, and a smaller portion of the illuminated area 154lies within the front surface 140 of the second optical member 146 b.Consequently, an irradiance distribution 144 c is obtained which is asuperposition of an increasing irradiance distribution 144 a and adecreasing irradiance distribution 144 b such that the irradiancedistribution 144 c. Thus it is possible, with the modulator units 260 asdescribed with reference to FIGS. 23 to 30, to produce all irradiancedistributions which have been shown in FIG. 22 and which are used suchthat an annular illumination setting continuously transforms into an Xdipole illumination setting along the X direction of the illuminatedfield 14.

What is claimed is:
 1. An illumination system, comprising: an opticalintegrator comprising an array of optical raster elements configured sothat, during use of the illumination system, a light beam is associatedwith each optical raster element; a condenser configured so that, duringuse of the illumination system, the condenser superimposes the lightbeams associated with the optical raster elements in a common fieldplane which is a mask plane or which is optically conjugate to the maskplane; a modulator configured so that, during use of the illuminationsystem, the modulator modifies a field dependency of an angularirradiance distribution in an illuminated field of the mask plane, themodulator comprising a plurality of modulator units, each modulator unitbeing: associated with one of the light beams; in front of the condenserso that only the associated light beam of the modulator unit impinges onthe modulator unit; and configured so that, during use of theillumination system, the modulator variably redistributes a spatialand/or an angular irradiance distribution of its associated light beam;and a control device configured so that, during use of the illuminationsystem, the control device controls the modulator units so that, duringuse of the illumination system: at least one modulator unitredistributes the spatial and/or the angular irradiance distribution ofan associated light beam if the control device receives an input commandthat the field dependency of the angular irradiance distribution in themask plane shall be modified; wherein a first angular irradiancedistribution is produced at a first portion of the illuminated field anda second angular irradiance distribution at a second portion of theilluminated field, wherein the second angular irradiance distribution isdistinct from the first angular irradiance distribution, and theillumination system is a microlithographic illumination system.
 2. Theillumination system of claim 1, wherein the first portion of theilluminated field is a two-dimensional area in which the first angularirradiance distribution is uniform, and the second portion of theilluminated field is a two-dimensional area in which the second angularirradiance distribution is uniform.
 3. The illumination system of claim1, wherein: the illuminated field has a long dimension in a firstdirection; the illuminated field has a short dimension in a seconddirection perpendicular to the first direction; the first portion of theilluminated field has at least one coordinate in the second direction incommon with the second portion of the illumination field; and the firstportion of the illuminated field has no coordinate in the firstdirection in common with the second portion of the illuminated field. 4.The illumination system of claim 1, wherein the first and the secondangular irradiance distributions are associated with illuminationsettings selected from the group consisting of conventional illuminationsettings, angular illumination settings, dipole illumination settings,and n-pole illumination settings with n≧4.
 5. The illumination system ofclaim 1, wherein the first and second angular irradiance distributionsare simultaneously produced.
 6. The illumination system of claim 1,wherein: each modulator unit is in a raster field plane located, in adirection of light propagation, in front of the array of optical rasterelements; and each modulator unit is configured so that, during use ofthe illumination system, the modulator variably redistributes thespatial irradiance distribution of the associated light beam in theraster field plane.
 7. The illumination system of claim 6, wherein eachmodulator unit is configured so that, during use of the illuminationsystem, the modulator unit shifts an area in the raster field plane,which is illuminated by the light beam associated with the modulatorunit, along a direction which is perpendicular to an optical axis of theillumination system.
 8. The illumination system of claim 7, wherein eachmodulator unit is configured so that, during use of the illuminationsystem, the modulator shifts the illuminated area without changing theangular irradiance distribution of the light beam.
 9. The illuminationsystem of claim 6, wherein: a first angular irradiance distribution isproduced at a first portion of the illuminated field and a secondangular irradiance distribution at a second portion of the illuminatedfield; the second angular irradiance distribution is distinct from thefirst angular irradiance distribution; and the first and second angularirradiance distributions are simultaneously produced, the illuminatedfield has a long dimension in a first direction; the illuminated fieldhas a short dimension in a second direction perpendicular to the firstdirection; the first portion of the illuminated field has at least onecoordinate in the second direction in common with the second portion ofthe illumination field; the first portion of the illuminated field hasno coordinate in the first direction in common with the second portionof the illuminated field; and the direction perpendicular to the opticalaxis of the illumination system is the first direction.
 10. Theillumination system of claim 1, wherein: each modulator unit is arrangedin or in close proximity to a pupil plane that is located, in adirection of light propagation, behind the array of optical rasterelements; and each modulator unit is configured so that, during use ofthe illumination system, the modulator unit variably redistributes, theangular irradiance distribution of the associated light beam in thepupil plane.
 11. The illumination system of claim 1, wherein eachmodulator unit comprises: an optical element configured so that, duringuse of the illumination system, the optical element changes thepropagation direction of the associated light beam impinging on theoptical element; and an actuator coupled to the optical element, whereinthe actuator is configured so that, during use of the illuminationsystem, the actuator changes the position and/or orientation of theoptical element in response to a control signal received from thecontrol device.
 12. The illumination system of claim 11, wherein theactuator is configured so that, during use of the illumination system,the actuator rotates the optical element around a rotational axis thatis inclined with respect to an optical axis of the illumination system.13. The illumination system of claim 1, wherein the modulator isconfigured so that, during use of the illumination system, the angularirradiance distribution discontinuously varies over the illuminatedfield.
 14. The illumination system of claim 1, wherein the modulator isconfigured so that, during use of the illumination system, the angularirradiance distribution continuously varies over at least a portion ofthe illuminated field.
 15. The illumination system of claim 14, wherein:a first angular irradiance distribution is produced at a first portionof the illuminated field and a second angular irradiance distribution ata second portion of the illuminated field; the second angular irradiancedistribution is distinct from the first angular irradiance distribution;the first portion of the illuminated field is a first line where thefirst angular irradiance distribution is uniform; the second portion ofthe illuminated field is a second line where the second angularirradiance distribution is uniform; and the modulator is configured sothat, during use of the illumination system, the first angularirradiance distribution continuously transforms into the second angularirradiance transforms within an area arranged between the first line andthe second line.
 16. The illumination system of claim 15, wherein thefirst line adjoins one end of the illuminated field and the second lineadjoins an opposite end of the illuminated field.
 17. The illuminationsystem of claim 16, wherein the one end and the opposite end delimit theilluminated field along an X direction of the illumination system. 18.An apparatus, comprising: an illumination system according to claim 1;and a projection objective, wherein the apparatus is a microlithographicprojection exposure apparatus.
 19. A method, the method comprising:providing of a microlithographic projection exposure system whichcomprises a projection objective and an illumination system of claim 1;defining a first desired angular irradiance distribution and a seconddesired angular irradiance distribution different from the first angularirradiance distribution; controlling the modulator units so that atleast one modulator unit redistributes the spatial and/or the angularirradiance distribution of an associated light beam, if the controldevice receives an input command that the field dependency of theangular irradiance distribution in the mask plane shall be modified. 20.An illumination system, comprising: an optical integrator comprising anarray of optical raster elements configured so that, during use of theillumination system, a light beam is associated with each optical rasterelement; a condenser configured so that, during use of the illuminationsystem, the condenser superimposes the light beams associated with theoptical raster elements in a common field plane which is a mask plane orwhich is optically conjugate to the mask plane; a modulator configuredso that, during use of the illumination system, the modulator modifies afield dependency of an angular irradiance distribution in an illuminatedfield of the mask plane, the modulator comprising a plurality ofmodulator units, each modulator unit being: associated with one of thelight beams; in front of the condenser so that only the associated lightbeam of the modulator unit impinges on the modulator unit; andconfigured so that, during use of the illumination system, the modulatorvariably redistributes a spatial and/or an angular irradiancedistribution of its associated light beam; and a control deviceconfigured so that, during use of the illumination system, the controldevice controls the modulator units so that, during use of theillumination system: at least one modulator unit redistributes thespatial and/or the angular irradiance distribution of an associatedlight beam if the control device receives an input command that thefield dependency of the angular irradiance distribution in the maskplane shall be modified; and wherein: a first angular irradiancedistribution is produced at a first portion of the illuminated field anda second angular irradiance distribution at a second portion of theilluminated field, wherein the second angular irradiance distribution isdistinct from the first angular irradiance distribution, and eachmodulator unit is configured so that, during use of the illuminationsystem, each modulator tilts the light beam associated with themodulator unit about a tilt axis which is inclined to an optical axis ofthe illumination system.
 21. An illumination system, comprising: anoptical integrator comprising an array of optical raster elementsconfigured so that, during use of the illumination system, a light beamis associated with each optical raster element; a condenser configuredso that, during use of the illumination system, the condensersuperimposes the light beams associated with the optical raster elementsin a common field plane which is a mask plane or which is opticallyconjugate to the mask plane; a modulator configured so that, during useof the illumination system, the modulator modifies a field dependency ofan angular irradiance distribution in an illuminated field of the maskplane, the modulator comprising a plurality of modulator units, eachmodulator unit being: associated with one of the light beams; in frontof the condenser so that only the associated light beam of the modulatorunit impinges on the modulator unit; and configured so that, during useof the illumination system, the modulator variably redistributes aspatial and/or an angular irradiance distribution of its associatedlight beam; and a control device configured so that, during use of theillumination system, the control device controls the modulator units sothat, during use of the illumination system: at least one modulator unitredistributes the spatial and/or the angular irradiance distribution ofan associated light beam if the control device receives an input commandthat the field dependency of the angular irradiance distribution in themask plane shall be modified; and a first angular irradiancedistribution is produced at a first portion of the illuminated field anda second angular irradiance distribution at a second portion of theilluminated field, the second angular irradiance distribution beingdistinct from the first angular irradiance distribution, wherein: theillumination system is a microlithographic illumination system; eachmodulator unit comprises: an optical element configured so that, duringuse of the illumination system, the optical element changes thepropagation direction of the associated light beam impinging on theoptical element; and an actuator coupled to the optical element; theactuator is configured so that, during use of the illumination system,the actuator changes the position and/or orientation of the opticalelement in response to a control signal received from the controldevice; and the actuator is configured so that, during use of theillumination system, the actuator rotates the optical element around arotational axis that is inclined with respect to an optical axis of theillumination system.